The present invention relates generally to a method of making an organic light-emitting device (OLED) and particularly to an improved method of forming solid pellets from powders of organic materials and using such pellets in physical vapor deposition to make an organic layer on a substrate which will form part of an OLED.
An organic light-emitting device, also referred to as an organic electroluminescent (EL) device, can be constructed by sandwiching two or more organic layers between first and second electrodes.
In a passive matrix OLED of conventional construction, a plurality of laterally spaced light-transmissive anodes, for example indium-tin-oxide (ITO) anodes, are formed as first electrodes on a light-transmissive substrate such as, for example, a glass substrate. Two or more organic layers are then formed successively by physical vapor deposition of respective organic materials from respective sources, within a chamber held at reduced pressure, typically less than 10xe2x88x923 Torr. A plurality of laterally spaced cathodes are deposited as second electrodes over an uppermost one of the organic layers. The cathodes are oriented at an angle, typically at a right angle, with respect to the anodes.
Such conventional passive matrix organic light-emitting devices are operated by applying an electrical potential (also referred to as a drive voltage) between appropriate columns (anodes) and, sequentially, each row (cathode). When a cathode is biased negatively with respect to an anode, light is emitted from a pixel defined by an overlap area of the cathode and the anode, and emitted light reaches an observer through the anode and the substrate.
In an active matrix organic light-emitting device (OLED), an array of anodes are provided as first electrodes by thin-film transistors (TFTs) which are connected to a respective light-transmissive portion. Two or more organic layers are formed successively by vapor deposition in a manner substantially equivalent to the construction of the aforementioned passive matrix device. A common cathode is deposited as a second electrode over an uppermost one of the organic layers. The construction and function of an active matrix organic light-emitting device is described in commonly-assigned U.S. Pat. No. 5,550,066, the disclosure of which is herein incorporated by reference.
Organic materials, thicknesses of vapor-deposited organic layers, and layer configurations, useful in constructing an organic light-emitting device, are described, for example, in commonly-assigned U.S. Pat. Nos. 4,356,429; 4,539,507; 4,720,432; and 4,769,292, the disclosures of which are herein incorporated by reference.
Organic materials useful in making OLEDs, for example organic hole-transporting materials, organic light-emitting materials predoped with an organic dopant, and organic electron-transporting materials can have relatively complex molecular substrates with relatively weak molecular bonding forces, so that care must be taken to avoid decomposition of the organic material(s) during physical vapor deposition.
The aforementioned organic materials are synthesized to a relatively high degree of purity, and are provided in the form of powders, flakes, or granules. Such powders or flakes have been used heretofore for placement into a physical vapor deposition source wherein heat is applied for forming a vapor by sublimation or vaporization of the organic material, the vapor condensing on a substrate to provide an organic layer thereon.
Several problems have been observed in using organic powders, flakes, or granules in physical vapor deposition:
(i) powders, flakes, or granules are difficult to handle because they can acquire electrostatic charges via a process referred to as triboelectric charging;
(ii) powders, flakes, or granules of organic materials generally have a relatively low physical density (expressed in terms of weight per unit volume) in a range from about 0.05 to about 0.2 g/cm3, compared to a physical density of an idealized solid organic material of about 1 g/cm3;
(iii) powders, flakes, or granules of organic materials have an undesirably low thermal conductivity, particularly when placed in a physical vapor deposition source which is disposed in a chamber evacuated to a reduced pressure as low as 10xe2x88x926 Torr. Consequently, powder particles, flakes, or granules are heated only by radiative heating from a heated source, and by conductive heating of particles or flakes directly in contact with heated surfaces of the source. Powder particles, flakes, or granules which are not in contact with heated surfaces of the source are not effectively heated by conductive heating due to a relatively low particle-to-particle contact area; and
(iv) powders, flakes, or granules can have a relatively high ratio of surface area to volume of such particles and a correspondingly high propensity to entrap air and/or moisture between particles under ambient conditions. Consequently, a charge of organic powders, flakes, or granules loaded into a physical vapor deposition source which is disposed in a chamber must be thoroughly outgased by preheating the source once the chamber has been evacuated to a reduced pressure. If outgasing is omitted or is incomplete, particles can be ejected from the source together with a vapor stream during physical vapor-depositing an organic layer on a substrate. An OLED, having multiple organic layers, can be or can become functionally inoperative if such layers include particles or particulates.
Each one, or a combination, of the aforementioned aspects of organic powders, flakes, or granules can lead to nonuniform heating of such organic materials in physical vapor deposition sources with attendant spatially nonuniform sublimation or vaporization of organic material and can, therefore, result in potentially nonuniform vapor-deposited organic layers formed on a substrate.
It is an object of the present invention to provide a method of handling organic material adaptable for making an organic layer on a substrate which will form part of an OLED.
It is another object of the present invention to provide a method of consolidating organic powder into a solid pellet.
It is a further object of the invention to provide a method of making an organic layer from a solid pellet of organic material and on a substrate which will form part of an OLED.
It is still a further object of the present invention to provide a method of consolidating into a solid pellet a mixture of a sublimable organic material powder and a thermally conductive non-sublimable ceramic powder.
In one aspect, the present invention provides a method of making an organic layer from an organic material on a substrate which will form part of an OLED, comprising the steps of:
(a) providing a sublimable organic material in a powder form;
(b) providing a thermally conductive and non-sublimable ceramic material in a powder form;
(c) forming a mixture of the sublimable organic material powder and thermally conductive and non-sublimable ceramic material powder;
(d) placing such mixture into a die and using two punches, a lower and an upper punch, to apply sufficient pressure to the mixture to cause the mixture of powders to consolidate into a solid pellet;
(e) applying heat to the die during or prior to applying pressure by the opposing punches to aid in causing the mixture of powders to consolidate into a solid pellet; and
(f) removing the pellet from the die.
In another aspect, the present invention provides a method of using the solid pellet formed for making a layer of organic material for an OLED device comprising the steps of:
(a) placing the solid pellet into a physical vapor deposition source disposed in a chamber;
(b) positioning the substrate in the chamber and in a spaced relationship with respect to the source;
(c) evacuating the chamber to a reduced pressure; and
(d) applying heat to the source to cause at least a portion of the organic material in the pellet to sublime while the thermally conductive ceramic material remains unsublimed to provide a vapor of the organic material which forms the organic layer on the substrate.
A feature of the present invention is that the method of consolidating organic powder into a solid pellet can be accomplished with relatively simple tools and at a location remote from a location of use of such pellet in a physical vapor deposition apparatus.
Another feature of the present invention is that the method of consolidating organic powder into a solid pellet substantially facilitates handling, transfer or shipping of organic material in and between different locations.
Another feature of the present invention is that a plurality of pellets of organic material, prepared by the method of the present invention, can be handled, transferred or shipped in a container having a significantly reduced volume with respect to a container for handling, transferring or shipping organic material in powder form and of comparable weight.
Another feature of the present invention is that a solid pellet of an OLED-material can be made by the method of the present invention wherein a powder of at least one OLED host-material and a powder of at least one organic dopant material are mixed or blended to provide a mixture prior to consolidating the mixture into a solid pellet.
Another feature of the present invention is that the method of consolidating powder into a solid pellet and the method of making an organic layer on a substrate by evaporating a portion of a solid pellet in a physical vapor deposition source substantially eliminates ejection of powder particles from the source and, accordingly, provides an organic layer substantially free from particulate inclusions.
Another feature of the present invention is that the method of consolidating powder into a solid pellet can be tailored to provide a pellet having a shape selected to conform with a shape of a physical vapor deposition source from which a portion of a pellet is evaporated to form an organic layer on a substrate.
Another feature of the present invention is that the homogeneous dispersion of thermally conductive ceramic powder in otherwise thermally non-conductive solid organic pellets helps the consolidation process and also thermal evaporation by means of distributing heat uniformly throughout the solid pellets.
Another important feature of the present invention is that the thermally conductive ceramic powder does not interfere or contaminate the thermally evaporated organic molecules.